Hole Doping Research Articles

Reconfigurable spin tunnel diodes by doping engineering VS2 monolayers.

We propose a reconfigurable spin tunnel diode based on a small spin-gapped semiconductor (non-doped VS2 monolayer) and semi-metallic magnets (doped VS2 monolayer) separated by a thin insulating tunneling barrier (h-BN). By using first-principles calculations assisted by the nonequilibrium Green's function method, we have carried out a comprehensive study of spin-dependent current and spin transport properties while varying the bias. The device exhibited a magnetization-controlled diode-like behavior with forward-allowed current under antiparallel magnetizations and reverse-forbidden current under parallel magnetizations at the two electrodes. The threshold voltage is tunable by the hole doping density of VS2 monolayers. The doping effect on VS2 monolayers indicates that the magnetic moments, the Heisenberg exchange parameters and Curie temperatures can be monotonically reduced by a larger hole doping density. Our study on VS2 heterostructures has presented a simple and practical device strategy with very promising applications in spintronics.

Cuprate superconducting materials above liquid nitrogen temperature from machine learning.

The superconductivity of cuprates remains a challenging topic in condensed matter physics, and the search for materials that superconduct electricity above liquid nitrogen temperature and even at room temperature is of great significance for future applications. Nowadays, with the advent of artificial intelligence, research approaches based on data science have achieved excellent results in material exploration. We investigated machine learning (ML) models by employing separately the element symbolic descriptor atomic feature set 1 (AFS-1) and a prior physics knowledge descriptor atomic feature set 2 (AFS-2). An analysis of the manifold in the hidden layer of the deep neural network (DNN) showed that cuprates still offer the greatest potential as superconducting candidates. By calculating the SHapley Additive exPlanations (SHAP) value, it is evident that the covalent bond length and hole doping concentration emerge as the crucial factors influencing the superconducting critical temperature (Tc). These findings align with our current understanding of the subject, emphasizing the significance of these specific physical quantities. In order to improve the robustness and practicability of our model, two types of descriptors were used to train the DNN. We also proposed the idea of cost-sensitive learning, predicted the sample in another dataset, and designed a virtual high-throughput search workflow.

Effect of EBL thickness on the performance of AlGaN deep ultraviolet light-emitting diodes with polarization-induced doping hole injection layer

Numerical simulation results reveal that introducing polarization-induced doping hole injection layer (HIL) in the DUV-LEDs results in a downward band-bending at the EBL/HIL interface, which causes more severe electron leakage. The effect of optimizing EBL thickness on the performance of DUV-LEDs was investigated to mitigate the side-effects due to the introduction of polarization-induced doping HIL. The experimental results show that as the EBL thickness increases, the external quantum efficiency (EQE) and wall-plug efficiency (WPE) of the DUV-LEDs increase and then decrease, while the efficiency droop in an opposite trend. The maximum EQE and WPE of 5.95% and 5.48%, respectively, were achieved when the EBL thickness was 20 nm. Meanwhile, increasing the EBL thickness can improve the DUV-LED reliability. However, an excessively thick EBL will deteriorate the reliability due to generating more Joule heat. Therefore, the reliability of DUV-LED was optimal when the EBL thickness was 25 nm.

Electronic Structure of InCo2As2 and KInCo4As4: LDA + DMFT

A comparative analysis of the electronic structure obtained in the DFT/LDA and LDA + DMFT approaches of the possible isostructural analogues of iron superconductors InCo2As2 and KInCo4As4 with the electronic structure of the parent high-temperature superconductor system BaFe2As2 is carried out. It is established that in spite of the rather large value of the electron-electron correlations (local Coulomb interaction on the Co-3d shell U = 4.0 eV, the Hund exchange interaction J = 0.85 eV), in the considered systems a relatively small quasiparticle mass renormalization 1.2–1.35 at the Fermi level is observed. The correlation effects lead to the remarkable shift and compression of the spectrum below –0.8 eV. The band structure of InCo2As2 near the Fermi level is qualitatively similar to the previously studied BaCo2As2, and differs significantly from the band structure of BaFe2As2. In the KInCo4As4 system, the bands near the Fermi level resemble the band structure of BaFe2As2, and the Fermi surfaces have a similar topology. This indirectly points to the possibility of superconductivity in KInCo4As4. Also according to the results of LDA + DMFT calculations it is seen that with a rather small hole or electron doping in the KInCo4As4 system will experience topological Lifshitz transitions. We believe that the synthesis of the InCo2As2 and KInCo4As4 compounds considered in this paper is important for the study of superconductivity in this class of materials.

Effect of external magnetic field and doping on electronic and thermodynamic properties of planer and buckled silicene monolayer

In this research, the electronic and thermodynamic properties of the planer and buckled silicene monolayer under an external magnetic field and doping using the tight-binding (TB) model and the Green function approach are investigated. Also, the dependence of the electronic heat capacity and magnetic susceptibility with temperature, external magnetic field, electron, and hole doping for the planer and buckled silicene monolayer is calculated. Our numerical calculation exhibits that the planer and buckled silicene monolayer have a zero band gap. We find that the electronic heat capacity increases (decreases) by applying an external magnetic field, and electron and hole doping at lower (higher) temperatures due to the increase in the thermal energy (scattering and collision) of the charge carriers. Finally, we observe that the planer and buckled silicene monolayer is antiferromagnetic, which is changed to the ferromagnetic phase when an external magnetic field and doping are applied, which makes the silicene monolayer suitable for spintronic applications.

Ab initio low-energy effective Hamiltonians for the high-temperature superconducting cuprates Bi2Sr2CuO6, Bi2Sr2CaCu2O8, HgBa2CuO4, and CaCuO2

We derive $ab$ $initio$ low-energy effective Hamiltonians (LEH) for high-temperature superconducting (SC) copper oxides Bi$_2$Sr$_2$CuO$_6$ (Bi2201, $N_{\ell}=1$, $T_c^{\rm exp} \sim 10$ K), Bi$_2$Sr$_2$CaCu$_2$O$_8$ (Bi2212, $N_{\ell}=2$, $T_c^{\rm exp} \sim 84$ K), HgBa$_2$CuO$_4$ (Hg1201, $N_{\ell}=1$, $T_c^{\rm exp} \sim 90$ K) and CaCuO$_2$ (Ca11, $N_{\ell}=\infty$, $T_c^{\rm exp} \sim 110$ K), with different experimental optimal SC transition temperature $T_c^{\rm exp}$ and number $N_{\ell}$ of laminated CuO$_2$ planes between the two neighboring block layers. We apply the latest methodology of the multiscale $ab$ $initio$ scheme for correlated electron systems (MACE), and focus on the LEH consisting of one antibonding (AB) Cu$3d_{x^2-y^2}$/O$2p_{\sigma}$ orbital centered on each Cu atom. We discuss prominent features of this LEH: (1) The ratio $U/|t_1|$ between the onsite effective Coulomb repulsion (ECR) $U$ and amplitude of nearest neighbour hopping $t_1$ increases with $T^{\rm exp}_c$ and $N_{\ell}$, consistently with the expected increase in $d$-wave SC correlation function $P_{dd}$ with $U/|t_1|$. One possible cause of the increase of $U/|t_1|$ is the replacement of apical O atoms by Cu atoms from neighbouring CuO$_2$ planes when $N_{\ell}$ increases. Furthermore, we show that the increase in distance between Cu and apical O atoms decreases the effective screening (ES) by electrons outside of the LEH and increases $U/|t_1|$. (2) For Hg1201 and Ca11, we show that $U/|t_1|$ decreases when hole doping per AB orbital $\delta$ increases, which may partly account for the disappearance of SC when $\delta$ exceeds the optimal value in experiment. (3) For $N_{\ell} \geq 2$, off-site inter-CuO$_2$ plane ECR is comparable to off-site intra-CuO$_2$ plane ECR. We discuss contributions of inter-CuO$_2$ plane ECR to both $P_{dd}$ and the stability of the SC state.

Theoretical prediction of two-dimensional BC2X (X = N, P, As) monolayers: ab initio investigations

In this work, novel two-dimensional BC_2X (X = N, P, As) monolayers with X atoms out of the B–C plane, are predicted by means of the density functional theory. The structural, electronic, optical, photocatalytic and thermoelectric properties of the BC_2X monolayers have been investigated. Stability evaluation of the BC_2X single-layers is carried out by phonon dispersion, ab-initio molecular dynamics (AIMD) simulation, elastic stability, and cohesive energies study. The mechanical properties reveal all monolayers considered are stable and have brittle nature. The band structure calculations using the HSE06 functional reveal that the BC_2N, BC_2P and BC_2As are semiconducting monolayers with indirect bandgaps of 2.68 eV, 1.77 eV and 1.21 eV, respectively. The absorption spectra demonstrate large absorption coefficients of the BC_2X monolayers in the ultraviolet range of electromagnetic spectrum. Furthermore, we disclose the BC_2N and BC_2P monolayers are potentially good candidates for photocatalytic water splitting. The electrical conductivity of BC_2X is very small and slightly increases by raising the temperature. Electron doping may yield greater electric productivity of the studied monolayers than hole doping, as indicated by the larger power factor in the n-doped region compared to the p-type region. These results suggest that BC_2X (X = N, P, As) monolayers represent a new promising class of 2DMs for electronic, optical and energy conversion systems.

Strongly overdoped La2−xSrxCuO4 : Evidence for Josephson-coupled grains of strongly correlated superconductor

The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) with $x\ensuremath{\ge}0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at ${T}_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at ${T}_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at ${T}_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to ${T}_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general.

Fermi arcs from dynamical variational Monte Carlo

Variational Monte Carlo is a many-body numerical method that scales well with system size. It has been extended to study the Green function only recently by Charlebois and Imada [Phys. Rev. X 10, 041023 (2020)]. Here we generalize the approach to systems with open boundary conditions in the absence of translational invariance. Removing these constraints permits the application of embedding techniques like cluster perturbation theory (CPT). Using this approach we provide robust evidence of the existence of Fermi arcs in the one-band Hubbard model, an enduring problem in the physics of the pseudogap in cuprate high-temperature superconductors. We study the behavior of the Fermi surface and of the density of states as a function of hole doping for clusters of up to 64 sites, well beyond the reach of modern exact diagonalization solvers. We observe that the technique reliably captures the transition from a Mott insulator at half filling to a pseudogap, evidenced by the formation of Fermi arcs, and finally to a metallic state at large doping. The ability to treat large clusters with quantum cluster methods helps to minimize potential finite-size effects and enables the study of systems with long-range orders, which will help extend the reach of these already powerful methods and provide important insights on the nature of various strongly correlated many-electron systems, including the high-${\mathrm{T}}_{c}$ cuprate superconductors.

Topological superconductivity from doping a triplet quantum spin liquid in a flat-band system

We explore superconductivity in strongly interacting electrons on a decorated honeycomb lattice (DHL). An easy-plane ferromagnetic interaction arises from spin-orbit coupling in the Mott insulating phase, which favors a triplet resonance valence bond spin liquid state. Hole doping leads to partial occupation of a flat band and to triplet superconductivity. The order parameter is highly sensitive to the doping level and the interaction parameters, with $p+ip$, $f$ and $p+f$ superconductivity found, as the flat band leads to instabilities in multiple channels. Typically, first order transitions separate different superconducting phases, but a second order transition separates two time reversal symmetry breaking $p+ip$ phases with different Chern numbers ($\nu=0$ and 1). The Majorana edge modes in the topological ($\nu=1$) superconductor are almost localized due to the strong electronic correlations in a system with a flat band at the Fermi level. This suggests that these modes could be useful for topological quantum computing. The `hybrid' $p+f$ state does not require two phase transitions as temperature is lowered. This is because the symmetry of the model is lowered in the $p$-wave phase, allowing arbitrary admixtures of $f$-wave basis functions as overtones. We show that the multiple sites per unit cell of the DHL, and hence multiple bands near the Fermi energy, lead to very different nodal structures in real and reciprocal space. We emphasize that this should be a generic feature of multi-site/multi-band superconductors.

Synergistic effect of covalent functionalization and intrinsic electric field on β-Ga2O3/graphene heterostructures

β-Ga2O3/graphene heterostructure engineering has been regarded as an effective method to improve the optoelectronic performance of the β-Ga2O3 device. Here, hydrogenation/fluorination covalent functionalized graphene (HC/FC) was employed, and the synergistic effect of covalent functionalization and intrinsic electric field (Ein) was introduced to further improve and understand the interfacial properties of the heterostructure. Under the covalent functionalization, type-II band alignment with UV-infrared dual-band absorption was found for β-Ga2O3/HC heterostructure, while reserved type-II band alignment with hole doping was realized for p-type β-Ga2O3/FC heterostructure. Upon introducing the synergistic effect of covalent functionalization and Ein for β-Ga2O3/hydro-fluorinated graphene (HCF) heterostructure, except for the above similar characters, both the band offsets and optical absorption are further enhanced in β-Ga2O3/HCF heterostructures. When the direction of intrinsic Ein points to the contact interface, the Fermi level of β-Ga2O3/F-HCF was much closer to the valence band of β-Ga2O3. It was thought that the synergistic effect of covalent functionalization and Ein was more beneficial to promote the application of p-type β-Ga2O3. These findings were deeply revealed by the band levels, electrostatic potential, and charge transfer introduced. Our results were expected to provide useful insight into the synergistic effect of covalent functionalization and intrinsic Ein as well as to enhance the application potential of β-Ga2O3/graphene-based optoelectronic devices.

Revealing the intrinsic transport properties of antiperovskite Sr3SnO thin films

A topologically non-trivial band structure and reports of superconductivity have motivated significant investigation into the transport properties of the antiperovskite oxide Sr3SnO. Phase-pure films of Sr3SnO can be grown by molecular beam epitaxy, but they do not have the required extremely high hole-doping densities (>1 × 1021 cm−3) for which superconductivity has been observed in bulk materials. To date, high hole-doping densities have been achieved via inducing strontium deficiency, which inevitably results in impurity phases. Here, we show that indium acts as an effective hole dopant in Sr3SnO and can be used to achieve high hole doping densities in stoichiometric films. Films with carrier densities as high as 1.5 × 1021 cm−3 remain non-superconducting. We, therefore, suggest that Sr3SnO is probably not an intrinsic superconductor. A second question addressed in this work is the measurement of the intrinsic electrical transport properties of Sr3SnO, given its rapid degradation in air. We show that even in inert atmospheres, reducing the time needed for establishing electrical contacts and protecting the Sr3SnO film result in improved electrical properties. We demonstrate low carrier density films (4 × 1018 cm−3) with carrier mobilities of 400 cm2 V−1 s−1 at 10 K.

Comparison of thermoelectric properties of sorted and unsorted semiconducting single-walled carbon nanotube free-standing sheets

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising materials for thermoelectric generation (TEG) because of their large theoretical Seebeck coefficient (S). In this study, to discuss superiority of s-SWCNTs for TEG devices, thermoelectric properties of free-standing s-SWCNT sheets were compared with unsorted SWCNT sheets. To obtain the highest power density, the films were doped with triethyloxonium hexachloroantimonate and 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazole as the hole and electron dopants, respectively. The doped s-SWCNT sheets exhibited higher S but lower electrical conductivity than those of the unsorted SWCNT sheets. Consequently, the power factor of the s-SWCNT sheets was lower than that of the unsorted SWCNT sheets.

High Entropy Approach to Engineer Strongly Correlated Functionalities in Manganites.

Technologically relevant strongly correlated phenomena such as colossal magnetoresistance (CMR) and metal-insulator transitions (MIT) exhibited by perovskite manganites are driven and enhanced by the coexistence of multiple competing magneto-electronic phases. Such magneto-electronic inhomogeneity is governed by the intrinsic lattice-charge-spin-orbital correlations, which, in turn, are conventionally tailored in manganites via chemical substitution, charge doping, or strain engineering. Alternately, the recently discovered high entropy oxides (HEOs), owing to the presence of multiple-principal cations on a given sub-lattice, exhibit indications of an inherent magneto-electronic phase separation encapsulated in a single crystallographic phase. Here, the high entropy (HE) concept is combined with standard property control by hole doping in a series of single-phase orthorhombic HE-manganites (HE-Mn), (Gd0.25 La0.25 Nd0.25 Sm0.25 )1- x Srx MnO3 (x=0-0.5). High-resolution transmission microscopy reveals hitherto-unknown lattice imperfections in HEOs: twins, stacking faults, and missing planes. Magnetometry and electrical measurements infer three distinct ground states-insulating antiferromagnetic, unpercolated metallic ferromagnetic, and long-range metallic ferromagnetic-coexisting or/and competing as a result of hole doping and multi-cation complexity. Consequently, CMR ≈1550% stemming from an MIT is observed in polycrystalline pellets, matching the best-known values for bulk conventional manganites. Hence, this initial case study highlights the potential for a synergetic development of strongly correlated oxides offered by the high entropy design approach.

Electronic phase transition in the Re3Ge7 endohedral cluster compound

${\mathrm{Re}}_{3}{\mathrm{Ge}}_{7}$ is a unique electronic material showing low-temperature metal--insulator-like phase transition. Furthermore, bulk superconductivity can be induced in ${\mathrm{Re}}_{3}{\mathrm{Ge}}_{7}$ by hole doping. Here, we present transport properties, temperature-dependent high-resolution powder x-ray diffraction study, and electronic structure of ${\mathrm{Re}}_{3}{\mathrm{Ge}}_{7}$ in the vicinity of the electronic phase transition. Electrical resistivity and heat capacity measurements confirm the anomaly at 58.5 K in zero magnetic field. The Seebeck coefficient changes its sign and exhibits a maximum below the transition temperature indicating substantial changes in the electronic structure. Temperature-dependent structural studies confirm the second-order nature of phase transition, where unit cell parameters show abrupt change at the transition temperature. An analysis of interatomic distances indicates that the shrinkage of the unit cell volume is due to the interatomic changes taking place in the second coordination sphere of Re atoms. Electronic structure calculations reveal strong $5d\ensuremath{-}4p$ hybridization of valence orbitals near the Fermi level, which is situated between two deep pseudogaps of the density of states. The transition to the semiconducting state is accompanied by the opening of a band gap at the Fermi level right above the narrow pocket of hybridized states.

Transition from paramagnetism to ferromagnetism through additional hole doping of non-magnetic antimony in iron doped tellurium alloy

We have found an enhancement in the magnetic ordering of tellurium as a result of doping it with iron along with an additional doping of a non-magnetic element antimony. A weak ferromagnetism is observed from the magnetization hysteresis which can pave the way for new kinds of magnetic semiconductors. Using the modified solid state approach, we synthesized bulk alloys of Fe-doped tellurium with co-doping of Sb having general form Fe0.05(Te)1−xSbx; x = 0 and 0.03 and analyzed the sample for their structural, electrical and magnetic properties. Electrical resistivity measurements with varying external magnetic field has been carried out and it shows semiconducting nature for both samples. The conduction mechanism in the high temperature region follows small polaron hopping (SPH) model whereas in the low temperature region, variable range hopping (VRH) model is found to fit the data. Traditionally, though tellurium is diamagnetic in nature, x = 0 sample presents itself as a paramagnetic material as evident from the magnetization measurements. On the other hand, x = 0.03 sample has a small hysteresis which is brought about by the substitution of Sb. A negative to positive crossover is observed in the magnetoresistance plot of both samples which can be co-related to transition from variable range hopping mechanism to thermally activated hopping mechanism.

Electronic State of T′-Pr1.3−xLa0.7CexCuO4 (x = 0.10) Studied by Compton Scattering

We have performed Compton scattering measurements on as-grown and reduced single crystals of the electron-doped T'-cuprate Pr1.3-xLa0.7CexCuO4 (x = 0.10) to investigate the effect of reduction annealing on the electronic state. The obtained results have revealed that the numbers of electrons in the O2p orbital in the Zhang-Rice singlet band, the Cu3dx2-y2 orbital, and the Cu3d3z2-r2 orbital are increased by reduction annealing. The increase in the number of Cu3d3z2-r2 electrons suggests the existence of Cu3d3z2-r2 holes in the as-grown single crystal. This is spectroscopic evidence of local hole doping into the Cu3d3z2-r2 orbital around the excess oxygen, as suggested by transport measurements [T. Adachi et al., J. Phys. Soc. Jpn. 82, 063713 (2013)].

Hole-doping induced ferromagnetism in 2D materials

Two-dimensional (2D) ferromagnetic materials are considered as promising candidates for the future generations of spintronic devices. Yet, 2D materials with intrinsic ferromagnetism are scarce. Hereby, high-throughput first-principles simulations are performed to screen 2D materials that present a non-magnetic to a ferromagnetic transition upon hole doping. A global evolutionary search is subsequently performed to identify alternative possible atomic structures of the eligible candidates, and 122 materials exhibiting a hole-doping induced ferromagnetism are identified. Their energetic and dynamic stability, as well as magnetic properties under hole doping are investigated systematically. Half of these 2D materials are metal halides, followed by chalcogenides, oxides, and nitrides, some of them having predicted Curie temperatures above 300 K. The exchange interactions responsible for the ferromagnetic order are also discussed. This work not only provides theoretical insights into hole-doped 2D ferromagnetic materials, but also enriches the family of 2D magnetic materials for possible spintronic applications.

Pairing properties of the t−t′−t″−J model

We study the pairing properties of the two-dimensional $t$-$t'$-$t''$-$J$ model, where $t'$ and $t''$ are second and third neighbor hoppings, at a doping level $x\approx0.1$. Recent studies of the $t$-$t'$-$J$ model find strong pairing for $t'>0$, associated with electron doping, but an absence of pairing for $t'<0$ associated with hole doping. This is in contrast to the cuprates, where the highest transition temperatures appear for hole doping. Model parameterizations for the cuprates estimate a $t''$ comparable to $t'$, which, in principle, might fix this discrepancy. However, we find that it does not; we observe a suppression of pairing for the hole-doped system ($t'0$) while for the electron-doped system ($t'>0, t''<0$) $d$-wave pairing is robust. Extended hoppings appear to be insufficient to make the one-band $t$-$t'$-$J$ model capable of describing the pairing in the hole doped system.

Effects of the on-site energy on the electronic response of Sr3(Ir1−xMnx)2O7

We investigated the doping and temperature evolutions of the optical response of Sr3(Ir1−xMnx)2O7 single crystals with 0 ≤ x ≤ 0.36 by utilizing infrared spectroscopy. Substitution of 3d transition metal Mn ions into Sr3Ir2O7 is expected to induce an insulator-to-metal transition via the decrease in the magnitude of the spin–orbit coupling and the hole doping. In sharp contrast, our data reveal the resilience of the spin–orbit coupling and the incoherent character of the charge transport. Upon Mn substitution, an incoherent in-gap excitation at about 0.25 eV appeared with the decrease in the strength of the optical transitions between the effective total angular momentum Jeff bands of the Ir ions. The resonance energies of the optical transitions between the Jeff bands which are directly proportional to the magnitude of the spin–orbit coupling hardly varied. In addition to these evolutions of the low-energy response, Mn substitution led to the emergence of a distinct high-energy optical excitation at about 1.2 eV which is larger than the resonance energies of the optical transitions between the Jeff bands. This observation indicates that the Mn 3d states are located away from the Ir 5d states in energy and that the large difference in the on-site energies of the transition metal ions is responsible for the incoherent charge transport and the robustness of the spin–orbit coupling. The effect of Mn substitution was also registered in the temperature dependence of the electronic response. The anomaly in the optical response of the parent compound observed at the antiferromagnetic transition temperature is notably suppressed in the Mn-doped compounds despite the persistence of the long-range antiferromagnetic ordering. The suppression of the spin-charge coupling could be related to charge disproportionation of the Ir ions.